martes, 22 de marzo de 2011

Blindsided by Ferocity Unleashed by a Fault

On a map of Japan that shows seismic hazards, the area around the prefecture of Fukushima is colored in green, signifying a fairly low risk, and yellow, denoting a fairly high one. But since Japan sits on the collision of several tectonic plates, almost all of the country lies in an earthquake-risk zone. Most scientists expected the next whopper to strike the higher-risk areas southwest of Fukushima, which are marked in orange and red.

“Compared to the rest of Japan, it looks pretty safe,” said Christopher H. Scholz, a seismologist at the Lamont-Doherty Earth Observatory at Columbia University, referring to the area hit worst by the quake on March 11. “If you were going to site a nuclear reactor, you would base it on a map like this.”

Records kept for the past 300 years indicated that every few decades, part of the Japan trench, an offshore fault to the east of Fukushima, would break, generating an earthquake around magnitude 7.5, perhaps up to magnitude 8.0. While earthquakes that large would be devastating in many parts of the world, the Japanese have diligently prepared for them with stringent building codes and sea walls that are meant to hold back quake-generated tsunamis.

Shinji Toda, a professor of geology at Kyoto University in Japan, said a government committee recently concluded that there was a 99 percent chance of a magnitude-7.5 earthquake in the next 30 years, and warned there was a possibility for an even larger magnitude-8.0 quake.

So much for planning. Although Japan’s foresight probably saved tens of thousands of lives, it could not prevent the vast destruction of a magnitude-9.0 temblor, which releases about 30 times as much energy as a magnitude-8.0 quake. It was the largest ever recorded in Japan, and tied for fourth largest in the world since 1900. Thirty-foot tsunamis washed over the sea walls and swept inland for miles. The death toll is expected to be more than 20,000, and nearly 500,000 are now in shelters.

“I was surprised,” Dr. Toda said. “Nobody expected magnitude 9.”

This was not the first time scientists have underestimated the ferocity of an earthquake fault. Many were also caught by surprise by the magnitude-9.1 quake in 2004 off Sumatra, which set off tsunamis radiating across the Indian Ocean, killing more than 200,000 people.

Sometimes, scientists are blindsided by earthquakes because they occur along undiscovered faults. The deadly earthquakes in New Zealand this year; in Haiti last year; in Northridge, Calif., in 1994; and in Santa Cruz, Calif., in 1989 all happened along faults that scientists were unaware of until the ground shook.

“It’s shameful, but we’ve barely scratched the surface,” said Ross Stein, a geophysicist with the United States Geological Survey. In California, for instance, scientists have cataloged 1,400 faults, yet for smaller earthquakes — magnitude 6.7 or less — about one in three still occur on previously unknown faults.

“Humbling,” Dr. Stein said.

That raises a worrisome question: How many major quakes are lurking in underestimated or unknown faults?

The basic dynamics of earthquakes have been understood for decades. Earth’s crust is broken into pieces — tectonic plates — which slide and collide. But the sliding is not always smooth. When the plates stick together, they begin to buckle. Stress builds until the ground breaks and jumps, releasing energy in the form of vibrations: an earthquake. Not surprisingly, places close to plate boundaries are beset by earthquakes, while those far from the boundaries are not earthquake-prone.

The largest earthquakes occur in subduction zones, places where an ocean plate collides with and slides under a continental plate, particularly around the edge of the Pacific Ocean.

But some subduction zones seemed to produce more large earthquakes than others. One explanation was offered in 1980, when Hiroo Kanamori of the California Institute of Technology and Larry J. Ruff, now at the University of Michigan, published a paper that said giant earthquakes occurred more often along ocean faults where the subducting ocean plates were geologically young. The younger plates, like those off Alaska and Chile, were warmer, less dense and harder to push down into the Earth’s mantle, their thinking went. Meanwhile, the older, colder and denser ocean plates like those off Java and the Marianas trench in the Pacific would sink more easily and not produce the giant catastrophic quakes.

And yet the Pacific plate off Japan is 130 million years old, one of the oldest, and it generated a magnitude-9.0 counterexample. “It is not nearly as straightforward as I thought in the beginning,” Dr. Kanamori said.

Dr. Scholz of Columbia said the recent quake in Japan fit with a theory that he and Jaime Campos of the University of Chile developed in 1995. By their theory, the colliding tectonic plates off Fukushima were stuck, and should have been producing earthquakes. But the absence of spectacular earthquakes in the near historic record disagreed with their theory, and led Dr. Scholz to believe that something unknown was relieving the stress.

“Now we know we were wrong about that” and right in the first place, he said. “It does agree with the theory.”